You might look into a MKII Escort steering rack. They are much narrower than Mustang II racks and available in LHD, front steer configuration. They are readily available from England in many different ratios. The bump steer should be much better than with the mustang rack.

This car is the epitome of a tastefully modified 510. I really like all of the attention to detail on the car.
As far as the steering box breather goes, I seem to recall that the threads were tapered pipe threads. When I had a steering box in my car, I used a NPT to AN threaded adapter with a short pigtail of stainless tubing on the end to allow the fluid to expand without spilling. With the stock breather in place I found that there was always a bit of fluid weeping out. This also won't melt like the stock plastic one when the headers get hot!

Sorry to hear that it didn't go totally to plan. Talking to a friend that was out there, it seems like the conditions were awesome and the lines were short! Shame that you guys weren't able to get a good run in. Next year is the year! (isn't that what everybody says?)

That is consistent with how mine is. I'm guessing 4th is the worst because it is basically a direct drive straight through the transmission, giving the harmonic a direct path to resonate through.
On a 240sx there is a rubber coupling in the driveshaft. My guess is this is there to help dampen the harmonics and quiet the transmission (along with the heavy flywheel, stock clutch, soft transmission and motor mounts, rubber mounted shifter...etc).
A few years ago I installed an ATI damper on my KA. I don't know if it was the placebo affect, but it felt like there was a noticeable difference in how smooth the engine ran. There may have been a small change in the transmission noise as well, but it didn't eliminate it all together.

The clutch also plays a roll in this, as the sprung hub helps dampen harmonics coming from the engine. My thoughts are that that stock clutch springs are tuned to the harmonics much better than an aftermarket clutch.
Because my engine maybe puts out 10% more HP than stock, I can get away with running the stock clutch...with the turbo though, you might be SOL.

I found that when I switched from an aftermarket clutch to a stock clutch it quieted down the transmission quite a bit. From some of the research I did on this matter, I think the noise is more related to gear chatter than bearings (although it really sounds like bearings when you're driving!) People theorize that there is a harmonic above 3,000 rpm that travels from the engine through the transmission and then is reflected back from the diff to the transmission. When off throttle and the gears are back loaded, the problem is exacerbated. Harmonics like these are the reason some OEM's use things like guibo's in the driveshaft and dual mass flywheels.
I am interested in hearing how you deal with this issue, as it is one of the things in my car that always nags at me.

I think I went through 3 transmissions and one rebuild before I finally found one that is pretty quiet and doesn't grind. I got really good at pulling the engine, but it was damn frustrating. I have pretty much resigned to the fact that the transmission will never be silent in my car. I think that the combination of little to no sound deadening, more solid mounts, a single piece drive shaft, and a lightweight flywheel make the transmission quite a bit louder that you would expect. I also found that the transmission was louder with an aftermarket clutch installed.